Such methods and systems are known in the art. It is especially known that the gain G of a scintillation detector system, comprising a scintillator, a photocathode and a light readout detector (LRT), mostly comprising a photomultiplier tube (PMT), together with an evaluation system, is subject to a change in gain over time. The gain change of the overall system is substantially effected by the gain change of the LRT. That gain change is due to environmental changes, i.e. a modification in temperature over time or other environmental factors.
In order to stabilize the gain of the LRT, it is known in the art to conduct several measurements over time and to compare the results. An initial or reference measurement may take place at beginning of the first measurement of nuclear radiation, for example using a calibration source with well-known energies of the emitted gamma radiation. The light signals, produced by the gamma radiation in the scintillator crystal, are proportional to the energy deposed in that crystal. The light signals do then hit the LRT, i.e. a photocathode, causing that photocathode to emit electrons which are collected by a PMT. The number of photons produced in the scintillator per energy unit is called absolute light yield. The percentage of photons converted to photoelectrons at the photocathode of a PMT is the quantum efficiency (QE). According to Knoll, Radiation Detection and measurement, 3rd Ed. 2000, page 269, a typical light yield for a detector system, using a standard NaI(Tl) crystal from which nearly all the light is collected, is in the order of 38 photons per keV energy loss. Geometric and the quantum efficiency losses finally yield 8 to 10 photoelectrons at the photo cathode.
A PMT comprises a series of dynodes and a final anode. The—usually very few—photoelectrons from the photocathode are accelerated towards the first dynode where they cause emission of a multitude of secondary electrons, being emitted from that first dynode. The number of secondary electrons emitted per primary incident electron is the overall multiplication factor δ. Those electrons are then accelerated to the next dynode, where their number is again multiplied typically by the same factor δ, those electrons being led to the next dynode and so on, until they finally reach the anode of the PMT. Given that δ is the same for all of the N dynodes, the resulting gain G is then proportional to δN (Πv=1N δp). At the anode a current signal is measured, being proportional to the charge of the multitude of electrons. That charge is proportional to the amount of light, generated in the scintillator and therefore proportional to the energy deposed by the gamma radiation in the scintillator. Similar considerations apply when the LRT makes use of an amplifier other than a PMT, like an Avalanche diode or a semiconductor photomultiplier.
The resulting charge signal is then further processed and usually stored in a multichannel analyzer (MCA), each channel of that MCA corresponding to a specific radiation energy, deposed in the scintillator crystal. An accumulation of such energy signals results in an energy spectrum, each line in that spectrum corresponding to a specific energy deposed in the detector system.
As the charge signal is in a known relation to the number of photoelectrons emitted from the photocathode, one could, in theory, count the number of photoelectrons emitted from the photocathode also. In real life measurements, this not possible, even if it is known from Bellamy et al., Nucl. Instr. and Meth. in Phys. Res. A 339 (1994) 468-476, that there are statistical methods to estimate the amount of photoelectrons other than by charge integration, namely by deconvoluting a PMT spectrum.
For most applications, it is of interest to obtain the best resolution in energy a system allows. One of the problems, leading to a decrease in energy resolution is the gain shift, which is to be avoided therefore.
In order to do so, it is known to measure the gain at different times, using gamma radiation with known energy. This gamma radiation with known energy may be emitted by a calibration source, or by another radiation source with known energies. The gain of the at least two measurements at different times is compared and the signals are corrected by the difference, therefore multiplying all signals by a so-called gain correction factor, thereby stabilizing the overall system.
It is also known to use artificial light pulses instead of light pulses, generated by the scintillation crystal following the absorption of radiation energy. Such an artificial light source may be an LED.
The disadvantage of all this known systems is that one has to know either a specific—constant—line (energy) in the spectrum to be measured or to use a calibration source, thereby interrupting the measurement from time to time. In addition, especially at high count rates, it may be difficult to obtain a stabilization spectrum at all.